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The basic rule: The rule that you follow most of the time is roughly the same as the rule in English: If a word ends in a consonant, add -es. If it ends in an unaccented vowel, simply add an -s. In Spanish, y is treated as a consonant for purposes of pluralization.
Orthographic changes: Some words follow the general rule in terms of pronunciation, but a change in either a letter or an accent mark is needed. In plurals of words that end in -z, the -z changes to -ces for the plural. And if the addition of -es would change which syllable gets the accent, an accent is either dropped or added.